1. Field of the Invention
The present invention generally relates to a refrigerating apparatus. More particularly, the present invention relates to a refrigerating apparatus which adopts a two-temperature evaporation system.
2. Description of the Prior Art
FIG. 1 is a schematic diagram showing an example of an open display cabinet using a refrigerating apparatus constituting the background of the invention. The display cabinet 1 includes a body 3 having an inner case 5 which is provided with a plurality of shelves 7. The body 3 is comprised of heat insulating material. A cooling air passage 9 is defined between an inner wall surface of the body 3 and the inner case 5. The body 3 and the inner case 5 comprise an opening in a front surface thereof. A cooling air issuing slit 11 is defined between the body 3 and the inner case 5 at an upper end of the front opening and a cooling air drawing slit 13 is defined between the body 3 and the inner case 5 at a lower end of the front opening. As a result, an air curtain 15 is provided between the cooling air issuing slit 11 and the cooling air drawing slit 13 and thus commodities stocked on the shelves can be prevented from being exposed to the atmosphere. A fan 17 is located in a bottom portion of the cooling air passage 9. A main or first cooler C1 is located in a lower vertical portion of the cooling air passage 9. A sub or second cooler C2 is also located in a bottom portion of the cooling air passage 9 and in an upstream portion relative to the first cooler C1. By way of an example, the surface temperature of the second cooler C2 is set to the vicinity of 0.degree. C. (but over 0.degree. C.) and the surface temperature of the first cooler C1 is nearly set to -10.degree. C. through -15.degree. C.
High temperature and moisture laden air drawn by the fan 17 through the air drawing slit 13 is cooled and the moisture contained therein is removed in the form of water by the second cooler C2. The resultant air containing less moisture is further cooled to a predetermined temperature, for example, approximately -5.degree. C., by the first cooler C1 provided in the downstream. The air cooled by the first cooler C1 is issued through the cooling air issuing slit 11 to cool the inside of the cabinet 1 to a predetermined temperature, for example about 0.degree. C. through 2.degree. C. The cooling air thus issued is again drawn from the air drawing slit 13 together with ambient air. In such a manner, the cooling air is circulated. The cooling air issued through the cooling air issuing slit 11 forms the air curtain 15 which prevents an ambient air from entering the inside of the cabinet 1.
In accordance with the open display cabinet 1 adopting a conventional two-temperature evaporation system as shown in FIG. 1, the number of times of defrosting the first cooler C1 can be reduced, because the air is fed to the first cooler C1 after removing the moisture contained in the air by means of the moisture removing action of the second cooler C2, and thus the accumulated amount of frost on the first cooler C1 is reduced. Accordingly, it brings about a beneficial effect that damage to the stocked commodities due to rise of the temperature in the inside of the cabinet 1 caused by such defrosting is diminished.
FIG. 2 and FIG. 3 are circuit diagrams showing an example of a refrigeration cycle adopting a two-temperature evaporation system which can be used in the open display cabinet shown in FIG. 1, respectively. Referring to FIG. 2, a refrigeration cycle is adapted such that a compressor 21, a condenser 22, a first expansion valve or pressure reducing means 23, a first cooler or evaporator 24, a second expansion valve or pressure reducing means 25 and a second cooler or evaporator 26 are connected in series. In the example of FIG. 2, the first evaporator 24 corresponds to the second cooler C2 of FIG. 1 and the second evaporator 26 corresponds to the first cooler C1.
In the example of FIG. 3, a dual refrigeration cycle is used wherein the respective refrigeration cycles are adapted such that a first compressor 31 (a second compressor 31'), a first condenser 32 (a second condenser 32'), first pressure reducing means 33 (second pressure reducing means 33') and a first evaporator 34 (a second evaporator 34') are connected in series. The first evaporator 34 of FIG. 3 corresponds to the first cooler C1 of FIG. 1 and the second evaporator 34' corresponds to the second cooler C2.
In the open display cabinet shown in FIG. 1, an evaporating pressure regulating valve (not shown) is usually used for the refrigeration cycle such as those shown in FIGS. 2 and 3, so as to always maintain the surface temperature of the second cooler C2 over 0.degree. C. The use of such an evaporating pressure regulating valve permits an evaporating pressure in a cooler or an evaporator to be always maintained constant. For that reason, when the temperature in the inside of the cabinet 1 is relatively low or the ambient temperature is relatively low and the temperature of the air drawn from the drawing slit 13 falls to near 0.degree. C., the difference between the surface temperature of the second cooler C2 and the temperature of the drawn air becomes very small and thus quantity of heat to be exchanged is reduced. As a result, the moisture removing action of the second cooler C2 is drastically reduced, which brought about a disadvantage that the amount of frost accumulated onto the first cooler C1 is increased.
Furthermore, in case that a mechanism is not provided for regulation of evaporating pressure in an evaporator such as an evaporating pressure regulating valve, a disadvantage has been brought about that efficiency of driving is lowered when the temperature in the inside of the cabinet or the ambient temperature is relatively low and the drawn air is relatively low. More particularly, when in case that the temperature of the air drawn through the drawing slit 13 is relatively high, the moisture contained in such high temperature air is condensed in the second cooler C2 and removed in the form of water since the surface temperature of the second cooler C2 is set to approximately 0.degree. C. However, since an evaporating temperature in the second cooler C2 falls when the temperature of the drawn air is relatively low, the surface temperature of the second cooler C2 goes below 0.degree. C. Consequently, a portion of moisture contained in the drawn air is frozen or frosted on a surface of the second cooler C2, which leads to blocking of the second cooler C2 and thus lowers the evaporating pressure therein, with the consequence of diminishing the efficiency of operation.
In addition, when the dual refrigeration cycle as shown in FIG. 3 is used, since the coefficient of performance in one refrigeration cycle including the second cooler C2 (34'), the evaporating temperature of which is higher than that of the other refrigeration cycle, the efficiency of driving is relatively high as a whole refrigerating system. However, even in such a case, if and when a refrigeration load is decreased, it is difficult to set the surface temperature of the second cooler C2 to an approximate 0.degree. C. As a result, the second cooler C2 is covered with frost in a manner similar to the example of FIG. 1, and consequently the evaporating temperature of the cooler C2 lowers. Accordingly, it is difficult to maintain efficient driving for a long time, and also maintain the range of the refrigeration load capable of efficiently operating a refrigeration cycle including an evaporator having a relatively high evaporating temperature.
Conventionally, in the open display cabinet such as shown in FIG. 1, frost is still accumulated onto the first cooler C1 even if moisture contained in the air is removed using the second cooler C2. Accordingly, the frost accumulated onto the first cooler C1 is necessary to be removed. According to a conventional defrosting system in a refrigerating apparatus, it is known to remove the frost accumulated onto a front surface of the first cooler C1 by energizing a heater which is provided in a front surface of the cooler or evaporator. Since the conventional defrosting system is of a system wherein air is heated and the frost is melted by means of action of heat conduction through convection of the heated air, not only does it take a long time to defrost, but also it takes much heat loss and thus expends much electric power. In addition, the above described heat loss heats a cooling air flowing through a cooling air passage more than necessary, which results in a rise in temperature of commodities stocked in the inside of the cabinet. Consequently, a problem arises that the quality of the commodities is deteriorated.